update installation guide for linux

Issue description

When installing Processing to Ubuntu 16.04 I noticed that it's not very straighforward to find information
on how to install processing. I had to go google "how to install processing" and then it linked me to the
"Getting Started" tutorial by Casey Reas and Ben Fry.

Tutorial by Casey Reas and Ben Fry does not mention the existance of the installation.sh script, which is imo super convenient for (perhaps Linux-) beginners. And it may be better practice to install / extract processing into a different location than home folder.

TL;DR: No intuitive way to find (step-by-step) installation guide. Outdated installation information in the "Getting Started" tutorial (at least for linux).

URL(s) of affected page(s)

https://www.processing.org/tutorials/gettingstarted/
https://processing.org/download/

Proposed fix

It may be a good idea to put installation steps (first chapter of Casey's and Ben's tutorial) directly to the Downloads page - so the user can't miss it. Or, looking at Arduino page: linking to the getting started tutorial and mentioning that there are also installation instructions.

Also the linux part should be updated: mention the use of "install.sh" and maybe it would be nice to place processing into /opt/ or /usr/local/ rather than the home folder? Here is a "short" answer about installation locations in linux.